The non-contact inspection of a surface is often most easily accomplished by probing or illuminating the surface with light from a light source and sensing its reflected component as illustrated in FIG. 1. Deviations from surface flatness and other surface irregularities can be detected by noticing distortions and discontinuities in the reflected image. The light source typically has a known structure (so-called structured lighting).
This technique of probing a surface with a structured light source is extremely sensitive. An article by Lippincott and Stark entitled "Optical-Digital Detection of Dents and Scratches on Specular Metal Surfaces", Aug. 15, 1982, v. 21 (16), APPLIED OPTICS, describes a technique of imaging a series of regularly spaced lines through reflection from a specular surface. Apparatus embodying related ideas are described in U.S. Pat. Nos. 4,792,232; 5,436,726; and 5,237,404.
Automatic processing of images of structured lighting is desirable in an industrial setting. U.S. Pat. Nos. 4,792,232; 4,629,319; and 4,686,374 describe the electronic analog and digital processing of such images. Optical Fourier plane processing (or optical/digital processing) of images of structure lighting is also natural where the images contain periodic structures. U.S. Pat. No. 4,629,319 discloses a panel surface inspection system including an infrared (i.e. IR) source in the form of a CO.sub.2 laser and an IR retro-reflective material to form an image on a pair of adjacent infrared detectors in a pyroelectric vidicon.